Double Free in junos flowd before 24.2R2-S3 leads to DoS
CVE-2026-21918 Published on January 15, 2026
Junos OS: SRX and MX Series: When TCP packets occur in a specific sequence flowd crashes
A Double Free vulnerability in the flow processing daemon (flowd) of Juniper Networks Junos OS on SRX and MX Series allows an unauthenticated, network-based attacker to cause a Denial-of-Service (DoS). On all SRX and MX Series platforms, when during TCP session establishment a specific sequence of packets is encountered a double free happens. This causes flowd to crash and the respective FPC to restart.
This issue affects Junos OS on SRX and MX Series:
* all versions before 22.4R3-S7,
* 23.2 versions before 23.2R2-S3,
* 23.4 versions before 23.4R2-S4,
* 24.2 versions before 24.2R2.
Vulnerability Analysis
CVE-2026-21918 is exploitable with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a high impact on availability.
Weakness Type
What is a Double-free Vulnerability?
The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations. When a program calls free() twice with the same argument, the program's memory management data structures become corrupted. This corruption can cause the program to crash or, in some circumstances, cause two later calls to malloc() to return the same pointer. If malloc() returns the same value twice and the program later gives the attacker control over the data that is written into this doubly-allocated memory, the program becomes vulnerable to a buffer overflow attack.
CVE-2026-21918 has been classified to as a Double-free vulnerability or weakness.
Products Associated with CVE-2026-21918
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Affected Versions
Juniper Networks Junos OS:- Before 22.4R3-S7 is affected.
- Version 23.2 and below 23.2R2-S3 is affected.
- Version 23.4 and below 23.4R2-S4 is affected.
- Version 24.2 and below 24.2R2 is affected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.